Known automatic transmissions for motor vehicles comprise predominantly torque converters in conjunction with stepped ratio epicyclic gearsets, which provide excellent take-up features and full power gearshifts.
There are, however, a number of drawbacks associated with such known transmissions, for example:
Efficiency
When multiplying torque, known automatic transmissions tend to be very inefficient, for instance, when a 3:1 stall torque unit is producing twice the input torque, its efficiency is, typically less than 70%. Such losses are acceptable on high power/weight ratio vehicles, e.g. large engined cars, since the condition is transient; whereas, with low power/weight ratio vehicles, e.g. trucks, the torque multiplication from the converter is required for extended periods when climbing steep hills, the consequential power losses being criticised by drivers and, also, demanding substantial oil cooling facilities.
Engine Braking
Most modern torque converters have a clutching facility which engages usually in only higher gears, to avoid slippage and heterodyning but which may provide limited engine (overrun) braking only in those gears. The incorporation of such a clutching facility in lower gears would induce shock loads during gearshifts and, as a possible consequence, damage to the transmission.
Furthermore, during overrun, an unclutched converter turbine will overrun the impeller which not only effectively reduces all gear ratios, including the already shallower first gear on automatics, but also may lead to overheating of the transmission fluid on prolonged downgrades.
Since engine braking is frequently required by trucks in lower gears, to avoid or reduce brake fade during steep descents and is often enhanced on diesel trucks by employing an exhaust brake, which is not available with conventional automatics, there is consequently more demand upon the basic power brake systems with torque converter-type automatic transmissions, which can only be alleviated by the incorporation of some form of retarder in the transmissions or separate retarder in addition, thereby resulting in extra costs.
Some alternatives to torque converter-type automatic transmissions are:
(a) electronically-controllable, stepped ratio, manually-shiftable gearboxes requiring electronic declutching during gearshifts and known as E.M.S. gearboxes; and
(b) continuously variable transmissions referred to hereinafter as "CVTs", which usually have torque/power capacity and ratio spread limitations and are less efficient than gearsets at transmitting power but which can facilitate optimum engine r.p.m. and maximum engine braking over comparatively large, speed ranges.